Boron carbide based sintered compact and method for preparation thereof

ABSTRACT

A boron carbide based sintered body having a four-point flexural strength of at least 400 MPa and a fracture toughness of at least 2.8 MPa·m 1/2 , which has the following two preferred embodiments. (1) A boron carbide-titanium diboride sintered body obtained by sintering a mixed powder of a B 4 C powder, a TiO 2  powder and a C powder while reacting them under a pressurized condition and comprising from 95 to 70 mol % of boron carbide and from 5 to 30 mol % of titanium diboride, wherein the boron carbide has a maximum particle diameter of at most 5 μm. (2) A boron carbide-chromium diboride sintered body containing from 10 to 25 mol % of CrB 2  in B 4 C, wherein the sintered body has a relative density of at least 90%, boron carbide particles in the sintered body have a maximum particle diameter of at most 100 μm, and the abundance ratio (area ratio) of boron carbide particles of from 10 to 100 μm to boron carbide particles having a particle diameter of at most 5 μm, is from 0.02 to 0.6.

TECHNICAL FIELD

The present invention relates to a boron carbide based sintered body,such as a boron carbide-titanium diboride sintered body or a boroncarbide-chromium diboride sintered body, having high density, four-pointflexural strength and fracture toughness, and a process for itsproduction.

BACKGROUND ART

In general, a boron carbide sintered body is expected to have a widerange of applications as a material having a light weight and highhardness and being excellent in abrasion resistance or corrosionresistance. At present, it is used, for example, for a sandblast nozzle,a wire drawing die or an extrusion die. However, on the other hand, sucha boron carbide sintered body has a drawback that it has low strength.For example, K. A. Schwetz, J. Solid State Chemistry, 133, 177-81 (1997)discloses preparation of boron carbide sintered bodies by HIP treatmentunder various sintering conditions, but a boron carbide sintered bodyhaving a flexural strength of at least 600 MPa has not yet beenobtained.

Further, V. Skorokhod, J. Material Science Letter, 19, 237-239 (2000)discloses that a mixture comprising a boron carbide (B₄C) powder, atitanium dioxide (TiO₂) powder and a carbon (C) powder, is sinteredunder a pressurized condition employing a hot press method whilereacting a part of boron carbide with titanium dioxide and carbon (seethe following reaction formula), to obtain a boron carbide-titaniumdiboride sintered body, whereby a four-point flexural strength of 621MPa is obtained.Reaction formula: B₄C+2TiO₂+3C→2TiB₂+4CO

However, in order to make it practically possible to use a boron carbidebased sintered body in a wide range of applications, it is desired todevelop a boron carbide based sintered body having a still higherfour-point flexural strength. However, as mentioned above, according tothe conventional methods, a boron carbide based sintered body having ahigh four-point flexural strength exceeding 621 MPa has not yet beenobtained.

Further, a boron carbide based sintered body is hardly sinterable andaccordingly, it is usually prepared by a hot press method. Thisproduction method hinders a common application of a boron carbide basedsintered body, since its production cost is high. Accordingly, it isbeing studied to prepare a boron carbide sintered body by heating(sintering) under a non-pressurized condition (a normal pressure method)instead of the hot press method. For example, in the above-mentionedprior art reference K. A. Schwetz, J. Solid State Chemistry, 133, 177-81(1997), carbon is added as a sintering-assisting agent, and a boroncarbide sintered body is prepared under a non-pressurized condition.However, such a method is not practically preferred, since it isnecessary to carry out sintering at an extremely high temperature of atleast 2150° C.

Further, a boron carbide sintered body has an extremely high hardness,whereby it can hardly be processed by a usual grinding/polishing method,and further, the electric conductivity of the boron carbide sinteredbody is low at a level of from 10 to 300 S/m, whereby there has been aproblem that the discharge processing is difficult.

As mentioned above, a boron carbide sintered body is hardly sinterableand hardly processable, and at present, it is practically used only inan extremely limited application.

Under these circumstances, the present inventors have conducted anextensive research with an aim to develop a new boron carbide basedsintered body which has a four-point flexural strength higher than theabove-mentioned four-point flexural strength of 621 MPa and which makesit possible to realize a wide range of applications, and as a result,have found it possible to accomplish the desired object by selecting aspecific material and by carrying out sintering treatment with aspecific composition and under a specific temperature condition.

Further, the present inventors have found it possible to obtain a boroncarbide based sintered body having excellent characteristics bypreparing a sintered body having a specific microstructure wherein ahighly electrically conductive chromium diboride phase forms a threedimensional network structure, by adding a predetermined amount ofchromium diboride to a boron carbide powder having a specific physicalproperty and carrying out liquid phase sintering under a non-pressurizedcondition to form a liquid phase of chromium diboride.

The present invention has been accomplished on the basis of the abovediscoveries.

Namely, it is an object of the present invention to provide a novelboron carbide based sintered body having a four-point flexural strengthof at least 400 MPa and a fracture toughness of at least 2.8MPa·m^(1/2).

Further, it is an object of the present invention to provide a boroncarbide-titanium diboride sintered body having a four-point flexuralstrength of at least 700 MPa, preferably at least 800 MPa and a fracturetoughness of at least 3.0 MPa·m^(1/2).

Further, it is an object of the present invention to provide a novelprocess for producing a boron carbide material which makes it possibleto produce a boron carbide based sintered body which has a high densityand having the fracture toughness improved, wherein the maximum particlediameter of boron carbide is at most 5 μm, the titanium diborideparticles are uniformly dispersed in the boron carbide matrix, and theagglomerated/dispersed state of titanium diboride particles is uniformand good.

Further, it is an object of the present invention to provide a boroncarbide based sintered body which has a relative density of at least90%, an electrical conductivity of at least 5×10² S/m, a four-pointflexural strength of at least 400 MPa and a fracture toughness of atleast 3.0 MPa·m^(1/2), and a process for producing it by sintering undera non-pressurized condition.

DISCLOSURE OF THE INVENTION

The gist of the present invention to solve the above problems, is asfollows.

-   (1) A boron carbide based sintered body characterized by having a    four point flexural strength of at least 400 MPa as measured in    accordance with JIS R1601 and a fracture toughness of at least 2.8    MPa·m^(1/2) as measured in accordance with JIS R1607-SEPB method.-   (2) The boron carbide based sintered body according to the above    (1), which is a boron carbide-titanium diboride sintered body    obtained by sintering a mixed powder of boron carbide (B₄C) powder,    titanium dioxide (TiO₂) powder and carbon (C) powder while reacting    them under a pressurized condition and which comprises from 95 to 70    mol% of boron carbide and from 5 to 30 mol% of titanium diboride,    wherein the boron carbide has a maximum particle diameter of at most    5 μm.-   (3) The boron carbide based sintered body according to the above (1)    or (2), wherein the four point flexural strength is at least 700    MPa.-   (4) The boron carbide based sintered body according to the above    (1), (2) or (3), wherein the four point flexural strength is at    least 800 MPa, and the fracture toughness is at least 3.0    MPa·m^(1/2).-   (5) A boron carbide based sintered body which is a boron    carbide-chromium diboride sintered body containing from 10 to 25    mol% of chromium diboride (CrB₂) in boron carbide (B₄C),    characterized in that the sintered body has a relative density of at    least 90%, boron carbide particles in the sintered body have a    maximum particle diameter of at most 100 μm, and the abundance ratio    (area ratio) of boron carbide particles of from 10 to 100 μm to    boron carbide particles having a particle diameter of at most 5 μm,    is from 0.02 to 0.6.-   (6) The boron carbide based sintered body according to the above    (5), which has an electric conductivity of at least 5×10² S/m.-   (7) The boron carbide based sintered body according to the above    (6), which has a four point flexural strength of at least 400 MPa    and a fracture toughness of at least 3.0 MPa·m^(1/2).-   (8) A process for producing a boron carbide based sintered body,    characterized by mixing a titanium dioxide powder having an average    particle diameter of less than 1 μm and a carbon powder having an    average particle diameter of less than 1 μm to a boron carbide    powder having a maximum particle diameter of at most 5 μm, an    average particle diameter of at most 1 μm and a specific surface    area of at least 10 m²/g, and sintering the mixture within a    temperature range of from 1900 to 2100° C. while reacting them under    a pressurized condition.-   (9) The process for producing a boron carbide based sintered body    according to the above (8), wherein the specific surface area of the    boron carbide powder is at least 16 m²/g, and the average particle    diameter of each of the titanium dioxide powder and the carbon    powder is less than 0.1 μm.-   (10) A process for producing a boron carbide based sintered body,    characterized by molding a raw material powder having from 10 to 25    mol% of a chromium diboride powder added and mixed to a boron    carbide powder having an average particle diameter (D₅₀) of at most    2 μm and a specific surface area of at least 10 m²/g, followed by    heating from 1950 to 2100° C. in a non-oxidizing atmosphere under a    non-pressurized condition.-   (11) A shock absorber made of the boron carbide based sintered body    as defined in any one of the above (1) to (7).-   (12) The shock absorber according to the above (11), wherein the    shock absorber is for a high velocity missile.-   (13) An abrasion resistant component made of the boron carbide based    sintered body as defined in any one of the above (1) to (7).

BEST MODE FOR CARRYING OUT THE INVENTION

Now, the present invention will be described in further detail.

The present invention is a boron carbide based sintered body having afour-point flexural strength of at least 400 MPa as measured inaccordance with JIS R1601 and a fracture toughness of at least 2.8MPa·m^(1/2), preferably at least 3.0 MPa·m^(1/2), as measured inaccordance with JIS R1607-SEPB method, which is a novel boron carbidebased sintered body. Such a boron carbide based sintered body having ahigh four-point flexural strength and a high fracture toughness isuseful in a wide range of applications for e.g. sliding components,cutting tools, bulletproof plates or novel abrasion resistant componentsby virtue of its properties and thus is industrially useful.

A boron carbide based sintered body according to a preferred embodimentof the present invention is a boron carbide-titanium diboride sinteredbody obtained by mixing a boron carbide powder having a specificproperty with a titanium dioxide powder and a carbon powder in aspecific composition and sintering them in a specific temperature rangeunder a pressurized condition while reacting a part of the boron carbidepowder with the titanium dioxide powder and the carbon powder inaccordance with the following reaction formula.B₄C+2TiO₂+3C→2TiB₂+4CO

The present inventors have conducted various experimental studies on aprocess for sintering boron carbide while utilizing the above reactionand as a result, have found that when specific materials are selectedfor use and sintering treatment is carried out with a specificcomposition and under a specific temperature condition, it is possibleto obtain a boron carbide-titanium diboride sintered body which has ahigh density and a specific microstructure, wherein the maximum particlediameter of boron carbide is at most 5 μm, titanium diboride particlesare uniformly dispersed in the boron carbide matrix, and theagglomerated/dispersed state of titanium diboride particles is uniformand good and that the sintered body has a four-point flexural strengthof at least 700 MPa which has not been obtained heretofore, and has highstrength characteristics.

The boron carbide-titanium diboride sintered body is a boroncarbide-titanium diboride sintered body obtained by sintering a mixedpowder comprising a boron carbide (B₄C) powder, a titanium dioxide(TiO₂) powder and a carbon (C) powder while reacting them in a specifictemperature range under a pressurized condition, and it comprises from95 to 70 mol% of boron carbide and from 5 to 30 mol% of titaniumdiboride and yet, the maximum particle diameter of the boron carbide isat most 5 μm.

The reason for specifying the compositional ratio of boron carbide andtitanium diboride in the above range, is that if titanium diboridepresent in the boron carbide-titanium diboride sintered body is lessthan 5 mol%, no adequate effect for improving the strength can beobtained, and if it exceeds 30 mol%, the density of the sintered bodytends to be higher than 3.0 g/cm³, whereby the light weight feature ofthe boron carbide based sintered body will be lost, and at the sametime, the hardness will be low.

Further, even in the range of the above compositional ratio, if themaximum particle diameter of boron carbide in the sintered body exceeds5 μm, it will be difficult to obtain one having a high strength. Whenthe above-mentioned specific compositional range and the specificmicrostructure are satisfied at the same time, it will be possible forthe first time to obtain a boron carbide-titanium diboride sintered bodyhaving a sufficiently high strength.

The sintered body of the present invention shows a high strength with afour-point flexural strength of at least 700 MPa when the aboveconditions are satisfied. Further, according to a result of the studiesmade by the present inventors, by selecting preferred conditions withrespect to the particle sizes of the boron carbide powder, the titaniumdioxide powder and the carbon powder to be used as the startingmaterials, such as selecting those having finer particle sizes, itbecomes possible to obtain a boron carbide-titanium diboride sinteredbody which has a four-point flexural strength of at least 800 MPa andyet has a high strength characteristic with a fracture toughness of atleast 3.0 MPa·m^(1/2).

The boron carbide-titanium diboride sintered body of the presentinvention is effective for prolonging the useful life when it is appliedto a conventional sandblast, a wire drawing die, an extrusion die, etc.,and has a remarkable characteristic which can not be expected with aconventional boron carbide based sintered body, such that it can besuitably applied to a wide range of applications which has heretoforebeen impossible.

In the process for producing a boron carbide-titanium diboride sinteredbody of the present invention, a boron carbide powder, a titaniumdioxide powder and a carbon powder having specific properties are usedas the starting materials, and they are mixed and sintered whilereacting them in a specific temperature range under a pressurizedcondition of e.g. a hot pressing method. It is thereby possible toobtain a boron carbide-titanium diboride sintered body having theabove-mentioned characteristics, which has a high density and whereinthe maximum particle size of boron carbide is at most 5 μm, titaniumdiboride particles are uniformly dispersed in the boron carbide matrix,the agglomerated/dispersed state of titanium diboride particles isuniform and good, and the fracture toughness is improved, by controllingthe particle sizes, the maximum particle sizes, the agglomerated states,and the dispersed state, of boron carbide particles and titaniumdiboride particles in the boron carbide-titanium diboride sintered body.

The boron carbide powder to be used in the present invention is onehaving an average particle diameter (D50) is at most 1 μm and a maximumparticle diameter of at most 5 μm, as measured by a laser diffractionscattering analyzer (Microtrac). If the average particle diameter (D50)is larger than 1 μm, the sinterability tends to be poor, and it becomesimpossible to obtain a dense sintered body within a temperature range offrom 1900 to 2100° C., and in order to densify such a material, it willbe required to adopt a higher sintering temperature so that grain growthis likely to take place. Consequently, the maximum diameter of the boroncarbide particles in the obtainable sintered body tends to exceed 5 μm,whereby it tends to be difficult to obtain a sintered body having a highfour-point flexural strength. Further, with respect to the specificsurface area (BET) of the boron carbide powder, a boron carbide powderhaving a specific surface area of at least 10 m²/g is preferablyselected for use, since its sinterability is good.

With respect to the titanium dioxide powder and the carbon powder to beused in the present invention, it is necessary to use fine powders inorder to carry out a uniform reaction during sintering, and they areones having an average particle diameter (D50) of less than 1 μm asmeasured by a laser diffraction scattering analyzer (Microtrac). If theaverage particle diameter (D50) is at least 1 μm, large titaniumdiboride particles will be formed in the sintered body, and such largeparticles will be starting points for fracture, whereby it becomesimpossible to obtain a sintered body having a high four-point flexuralstrength.

Further, in a case where the average particle diameter is less than 0.1μm, it tends to be difficult to carry out the measurement accurately,since the powder tends to agglomerate during the measurement by a laserdiffraction scattering analyzer. Therefore, a BET average particlediameter calculated from the specific surface area may be employed.Further, titanium dioxide has crystal systems of rutile type, anatasetype and brookite type, and any type may be employed.

The boron carbide powder, the titanium dioxide powder and the carbonpowder having the above-mentioned physical properties may respectivelybe obtained by such a means as sieving, separation by sedimentation,pulverization, etc. Commercial products may be used so long as they havethe above-mentioned physical properties.

In the present invention, a titanium dioxide powder having an averageparticle diameter of less than 1 μm and a carbon powder having anaverage particle diameter of less than 1 μm are blended to a boroncarbide powder having an average particle diameter of at most 1 μm, amaximum particle diameter of at most 5 μm and a specific surface area ofat least 10 m²/g, preferably in a blend ratio of from 4.5 to 19 mol% ofthe titanium dioxide powder, and the molar ratio of carbonpowder/titanium dioxide powder being from 1.4 to 1.7, followed bymixing, so that the composition of the boron carbide-titanium diboridesintered body to be prepared would be from 95 to 70 mol% of boroncarbide and from 5 to 30 mol% of titanium diboride. Then, if necessary,this mixture is molded and then the above mixed powder or molded productis sintered in a temperature range of from 1900 to 2100° C. in vacuum orin an inert gas atmosphere of e.g. Ar while reacting them under apressurized condition to let titanium diboride particles form amongboron carbide particles, to prepare a dense boron carbide-titaniumdiboride sintered body having a relative density of at least 98%.

Here, in the method for obtaining the boron carbide-titanium diboridesintered body by sintering the mixed powder comprising the boron carbidepowder, the titanium dioxide powder and the carbon powder, in thespecific temperature range while reacting them under a pressurizedcondition, according to the study by the present inventors, there is atechnical problem that titanium diboride particles in the prepared boroncarbide-titanium diboride sintered body, tend to agglomerate in thereaction process and are likely to form large agglomerated masses, andif titanium diboride agglomerated masses or coarse boron carbideparticles larger than 5 μm, are present, they serve as fracture startingpoints and bring about deterioration of the four-point bending strength.

According to the present invention, a boron carbide powder having aspecific physical property is used, whereby sinterability of the boroncarbide powder itself is good, and as compared with formation oftitanium diboride particles, sintering among the boron carbide particleswill preferentially proceed, whereby titanium diboride particles will beuniformly dispersed in the boron carbide matrix, whereby theagglomerated/dispersed state of titanium diboride particles will beuniform and good. As a result, it is possible to make that there will beno substantial presence of coagulated particles of titanium diboride.Further, the maximum particle diameter of boron carbide is at most 5 μm,and accordingly, coarse boron carbide particles are not present from thebeginning. As a result, the obtainable boron carbide-titanium diboridesintered body has a four point flexural strength as high as at least 700MPa, as mentioned above.

In addition, according to the present invention, when a boron carbidepowder having an average particle size of at most 1 μm, a maximumparticle size of at most 5 μm and a specific surface area of at least 16m²/g is used, and a titanium dioxide powder having an average particlesize of less than 0.1 μm and a carbon powder having an average particlesize of less than 0.1 μm are employed, the agglomerated/dispersed stateof titanium diboride particles becomes more uniform and good. And, evenif particles of the titanium dioxide powder are joined during theprocess wherein sintering of the boron carbide powder proceeds for agrain growth to from 2 to 3 μm, titanium diboride particles of from 2 to3 μm will form, and the titanium diboride particles will be uniformlydispersed without being coagulated at all. As a result, boroncarbide-titanium diboride sintered body can be obtained, which has aspecific microstructure having titanium diboride uniformly dispersed andwhich has high strength.

Namely, in a boron carbide-titanium diboride sintered body, the thermalexpansion coefficient of titanium diboride is larger than boron carbide.Accordingly, in a case where titanium diboride particles having a sizeof from 2 to 3 μm are present in the boron carbide matrix, crackpropagation detour or microcracking takes place in the vicinity of theinterface between the boron carbide matrix and the titanium diborideparticles during the progress of fracture, whereby the fracturetoughness will be improved. And, in the process for producing a boroncarbide-titanium diboride sintered body, the agglomerated/dispersedstate of titanium diboride particles will be good, and the fracturetoughness will be improved. Its strength will further be improved, andit is possible to prepare a boron carbide-titanium diboride sinteredbody having a high flexural strength of at least 800 MPa and having afracture toughness of at least 3.0 MPa·m^(1/2).

In the present invention, the titanium dioxide powder having an averageparticle size of less than 0.1 μm, may be any one so long as theabove-mentioned requirements are satisfied. However, a spherical powderprepared by a vapor phase method will suitably be employed. Further, asthe carbon powder, any one may be used so long as the average particlediameter is less than 0.1 μm, and carbon black or acetylene black can bepreferably employed.

In the present invention, with respect to the sintering conditions, ifthe sintering temperature is lower than 1900° C., it will be difficultto prepare a sufficiently dense boron carbide-titanium diboride sinteredbody. On the other hand, if the sintering temperature is higher than2100° C., a fine sintered structure can not be obtained due to anabnormal grain growth, whereby the flexural strength is likely to below. Accordingly, it is preferred to select the temperature within arange of from 1900 to 2100° C.

Further, the pressure during the sintering is usually from 20 MPa to 100MPa, preferably from 30 MPa to 60 MPa. However, in a case where thepressure during the sintering is lower than 20 MPa, no adequately densesintered body can be obtained. Further, in a case where the pressureexceeds 100 MPa, discharge of a carbon monoxide gas to the exterior willbe prevented, whereby formation of titanium diborate is likely to beimpaired.

Another preferred boron carbide based sintered body of the presentinvention is a boron carbide-chromium diboride sintered body containingfrom 10 to 25 mol% of chromium diboride (CrB₂) in boron carbide (B₄C),characterized in that the sintered body has a relative density of atleast 90%, boron carbide particles in the sintered body have a maximumparticle diameter of at most 100 μm, and the abundance ratio (arearatio) of boron carbide particles of from 10 to 100 μm to boron carbideparticles having a particle diameter of at most 5 μm, is from 0.02 to0.6.

In order to prepare a dense boron carbide based sintered body bysintering under a non-pressurized condition, grain growth of boroncarbide to some extent is required, and if no grain growth takes place,a sintered body having a high density can not be obtained. On the otherhand, if grain growth proceeds too much, coarse grains will hinderdensification, whereby the density of the sintered body tends to ratherdecrease, and coarse particles will be starting points for fracture,whereby the flexural strength tends to decrease.

In the present invention, by using a boron carbide powder havingspecific physical properties, sintering is carried out under a specificnon-pressurized condition in a temperature range where a liquid phasecontaining chromium diboride (CrB₂) as the main component will form,whereby it is possible to obtain a boron carbide-chromium diboridesintered body characterized in that the maximum particle diameter ofboron carbide particles is at most 100 μm, the abundance ratio (arearatio) of boron carbide particles of from 10 to 100 μm to boron carbideparticles having a particle diameter of at most 5 μm, is within a rangeof from 0.02 to 0.6, the relative density is at least 90%, a highlyelectrically conductive chromium diboride phase forms a networkstructure three dimensionally, and the body has an electricalconductivity of at least 5×10² S/m, a four-point flexural strength of atleast 400 MPa and a fracture toughness of at least 3.0 MPa·m^(1/2).

The boron carbide powder to be used in the present invention maypreferably one having an average particle diameter (D₅₀) of at most 2 μmas measured by a laser diffraction scattering method or a Dopplermethod. If the average particle diameter (D₅₀) is larger than 2 μm, thesinterability tends to be poor, a dense sintered body can hardly beobtainable within a temperature range of from 1950 to 2100° C., and inorder to densify it, it will be required to sinter it at a highertemperature at which grain growth is more likely to take place, wherebydeterioration of the flexural strength is likely to be brought about.With respect to the specific surface area (BET), it is preferred toemploy a boron carbide powder having a specific surface area of at least10 m²/g, more preferably at least 15 m²/g, which has good sinterability.

The boron carbide powder having the above physical properties can beprepared by a means such as sieving, separation by sedimentation,pulverization, etc., but a commercial product having such physicalproperties may be available for use.

To the boron carbide powder having the above physical properties, from10 to 25 mol% of a chromium diboride powder is added and molded,followed by heating (sintering) in vacuum or under a non-pressurizedcondition under a non-oxidizing atmosphere such as Ar within a sinteringtemperature range of from 1950 to 2100° C. in a state where a chromiumdiboride based liquid phase is formed.

The chromium diboride powder will react and melt with a part of theboron carbide powder during the sintering to form a chromium diboridebased liquid phase, which will penetrate among boron carbide particles,and as compared with the boron carbide powder, it may be used even inthe form of a starting material powder having a large particle size.Preferably, a chromium diboride powder having an average particle size(D₅₀) of at most 8 μm may be used, and more preferably, one having anaverage particle diameter (D₅₀) of at most 4 μm may be used.

In a case where the sintering temperature is lower than 1950° C., achromium diboride based liquid phase will not be formed, whereby asufficiently dense boron carbide sintered body can not be prepared, anda three dimensional network structure of the chromium diboride phase cannot be formed, whereby high electrical conductivity can not be obtained.On the other hand, at a sintering temperature higher than 2100° C.,coarse boron carbide particles will be formed by grain growth, thusleading to deterioration of the flexural strength.

If the amount of chromium diboride is less than 10 mol%, a sufficientamount of the chromium diboride based liquid phase will not be formed,whereby a dense sintered body can hardly be obtained, and the effectsfor improving the electrical conductivity and the fracture toughnesstend to be inadequate. On the other hand, if the amount of chromiumdiboride exceeds 25 mol%, the density of the sintered body will behigher than 3.0 g/cm³, whereby the feature of light weight of the boroncarbide type sintered body will be impaired, and the hardness will alsobe low.

The boron carbide-chromium diboride sintered body of the presentinvention has excellent properties and is useful as an abrasionresistant component. In the present invention, the abrasion resistantcomponent means to include every type of a component such as a slidingcomponent, a cutting tool, an abrasion resistant part, etc.

Effects

The effect mechanism with the boron carbide-titanium diboride sinteredbody as a preferred embodiment of the present invention, is as follows.Usually, in a process for producing a boron carbide-titanium diboridesintered body by sintering a mixed powder comprising a boron carbidepowder, a titanium dioxide powder and a carbon powder while reactingthem under a pressurized condition, titanium diboride particles arelikely to agglomerate to form large agglomerated blocks in the reactionprocess, and if titanium diboride agglomerated blocks or coarse boroncarbide particles larger than 5 μm, are present, they are likely to actas starting points for fracture and bring about deterioration of thefour-point flexural strength.

However, in the present invention, the boron carbide-titanium diboridesintered body is prepared by using raw material powders havingprescribed properties in a prescribed blend ratio to obtain a prescribedcompositional ratio, whereby titanium diboride particles will beuniformly dispersed in the boron carbide matrix, and theiragglomerated/dispersed state is uniform and good, and as a result, aboron carbide-titanium diboride sintered body having high strength and aspecific microstructure wherein titanium diboride particles areuniformly dispersed in the boron carbide matrix, can be obtained.

Further, in the present invention, when a boron carbide powder having anaverage particle diameter of at most 1 μm, a maximum particle diameterof at most 5 μm and a specific surface area of at least 16 m²/g, isused, and a titanium dioxide powder having an average particle diameterof less than 0.1 μm and a carbon powder having an average particlediameter of less than 0.1 μm are used, the agglomerated/dispersed stateof titanium diboride particles will be more uniform and good, andconsequently, a boron carbide-titanium diboride sintered body having ahigh strength can be obtained which has a microstructure whereintitanium diboride is uniformly dispersed and which has its strengthfurther improved.

The effect mechanism with the boron carbide-chromium diboride sinteredbody as another preferred embodiment of the present invention, is asfollows. By carrying out liquid phase sintering under a non-pressurizedcondition to form a liquid phase of chromium diboride, to prepare asintered body having a specific microstructure wherein a highlyelectrically conductive chromium diboride phase forms a networkstructure three dimensionally, it is possible to prepare a boroncarbide-chromium diboride sintered body having excellentcharacteristics.

With the boron carbide-chromium diboride sintered body of the presentinvention, since the thermal expansion coefficient of chromium diborideis larger than boron carbide, cracking propagation detour ormicrocracking takes place in the vicinity of the interface between theboron carbide particles and the chromium diboride phase during theprogress of the fracture, whereby the fracture toughness will beimproved. Further, the maximum particle size is at most 100 μm,protruded portions of boron carbide particles will be diminished by thedissolution/precipitation mechanism of the chromium diboride basedliquid phase, whereby the stress concentration will be relaxed, andboron carbide particles will be bonded by the chromium diboride phase,whereby falling off of boron carbide particles during processing will besuppressed, and the fracture toughness will be improved, whereby thestrength will be improved, and a high flexural strength of at least 400MPa can be obtained.

Now, the present invention will be described in further detail withreference to Examples and Comparative Examples. However, it should beunderstood that the present invention is by no means restricted by thefollowing Examples, etc. The four-point flexural strength and thefracture toughness of the boron carbide based sintered bodies weremeasured by JIS R1601 and JIS R1607, respectively.

EXAMPLES 1 to 40

As boron carbide powders, specific boron carbide powders A, B and Chaving the physical properties as identified in Table 1, were employed.As a submicron-size titanium dioxide powder, one having an averageparticle diameter (D50 as measured by a laser diffraction scatteringanalyzer) of 0.3 μm and a crystal phase of rutile type, was used.Further, as a nano-size titanium dioxide powder, a spherical powderprepared by a gas phase method and having a specific surface area (BET)of 48.5 m²/g, an average particle diameter (BET method) of 31 nm and acrystal phase of 80% anatase and 20% rutile, was used. As a carbonpowder, carbon black having a specific surface area (BET) of 88.1 m²/gand an average particle diameter (BET method) of 30 nm, was used. TABLE1 Physical properties of boron carbide powders B₄C starting AverageMaximum material particle particle BET powder diameter μm diameter μmm²/g A 0.50 2.4 21.5 B 0.44 3.3 15.5 C 0.41 2.3 22.5 D 0.55 5.7 18.7 E1.20 5.9 8.6

To the boron carbide powder, 14.5 mol% of the submicron-size ornano-size titanium dioxide powder and 21.5 mol% of carbon black wereincorporated, and using a methanol solvent, mixing was carried out by aplanetary ball mill made of silicon carbide (SiC) at a rotational speedof 270 rpm for 1 hour, followed by drying by a evaporator and further bydrying at 150° C. for 24 hours. Then, the mixture was sieved through asieve with an opening of 250 μm to obtain a boron carbide-titaniumdioxide-carbon mixed powder.

Then, in a die made of graphite, the boron carbide-titaniumdioxide-carbon mixed powder was filled and molded under 7.5 MPa and thenplaced in a firing furnace. In a pressurized state at 5 MPa, heating wascarried out at a temperature raising rate of 40° C./min while vacuumingto a pressure of from 2.0×10⁻¹ to 2.0×10⁻² Pa by means of a diffusionpump. When the temperature reached 1000° C., vacuuming was terminated,and Ar gas was introduced at a flow rate of 2 liters/min to anatmosphere with a gas pressure of 0.103 MPa, followed by heating to1500° C. From 1500° C. to 2000° C., the temperature was raised at a rateof 10° C./min. After the temperature reached 2000° C., the pressure wasraised to 50 MPa and maintained for 1 hour to obtain a boron carbide-20mol% titanium diboride sintered body.

The surface of a test piece was finished by a surface grinding machineNo. 400. Further, the density of the test piece was measured by anArchimedes method, and the relative density was calculated. The surfaceof the test piece was subjected to lapping and etching treatment,whereupon SEM observation was carried out to obtain the maximum particlediameter of boron carbide. Further, by the X-ray diffraction method,identification of the crystal phase in the sintered body was carriedout. The results of such measurements are shown in Table 2. TABLE 2Examples and Comparative Examples Maximum Four- particle B₄C TiO₂Density of Relative point diameter starting starting sintered density offlexural Fracture of boron material material body sintered strengthtoughness carbide No. powder powder g/cm³ body % MPa MPa · m^(1/2) μmExample 1 A Submicron 2.82 100 720 3.1 3.5 Example 2 B Nano size 2.82100 720 2.8 3.4 Example 3 A Nano size 2.82 100 870 3.4 3.8 Example 4 CNano size 2.82 100 815 3.2 3.9 Comparative E Submicron 2.75 97.8 475 2.86.4 1 Comparative D Nano size 2.82 100 585 2.8 6.1 2

Each of the boron carbide-titanium diboride sintered bodies prepared inExamples 1 to 4 had a high density and a maximum particle diameter ofboron carbide of at most 5 μm and a high four-point flexural strength ofat least 700 MPa. Especially, in Examples 3 and 4, a four-point flexuralstrength of at least 800 MPa and a high fracture toughness of at least 3MPa·m^(1/2) were obtained. Further, in each sintered body, a crystalphase was detected with respect to boron carbide and titanium diboride,and unreacted titanium dioxide was not detected.

Comparative Examples 1 and 2

Then, as Comparative Examples, boron carbide-20 mol% titanium diboridesintered bodies were prepared in the same manner as in Examples 1 to 4except that the composition was changed to a combination of the boroncarbide powder E as identified in Table 1 and the submicron-sizetitanium dioxide powder as used in Examples 1 to 4, and a combination ofthe boron carbide powder D as identified in Table 1 and the nano-sizetitanium dioxide powder.

Further, in the same manner as in Examples 1 to 4, evaluation of thefour-point flexural strength, the fracture toughness, the density of thesintered body and the maximum particle diameter of boron carbide, wascarried out. The results of measurements thereof are shown in Table 2.The four-point flexural strength of the sintered body in each ofComparative Examples 1 and 2, was low at a level of not more than 600MPa, and the maximum particle diameter of boron carbide was larger than5 μm.

EXAMPLE 5

To a boron carbide powder I having the physical properties as identifiedin Table 3, 20 mol% of a chromium diboride powder having an averageparticle diameter (D₅₀) of 3.5 μm was blended, and using a methanolsolvent, the blend was mixed by a planetary ball mill made of SiC at arotational speed of 275 rpm for 1 hour. The slurry was dried by anevaporator and further dried at 150° C. for 24 hours, and then it wassieved through a sieve of 250 mesh to obtain a boron carbide-chromiumdiboride mixed powder.

This powder was molded in a mold under 20 MPa, followed by CIP moldingunder 200 MPa to obtain a molded product. The molded product was putinto a graphite container and placed in a resistance heating type firingfurnace. Heating was carried out at a temperature-raising rate of 40°C./min while vacuuming to a pressure of from 2.0×10⁻¹ to 2.0×10⁻² Pa bymeans of a diffusion pump. When the temperature reached 1000° C.,vacuuming was terminated, and Ar gas was introduced, followed by heatingto 1500° C. From 1500° C. to 2030° C., heating was carried out at atemperature raising rate of 10° C./min. After the temperature reached2030° C., sintering was carried out for 1 hour under a non-pressurizedcondition to obtain a boron carbide-chromium diboride sintered body.TABLE 3 Physical properties of B₄C starting material powders Average B₄Cstarting particle BET material powder diameter μm m²/g I 0.43 15.3 II1.60 17.5 III 2.90 8.6

The surface of a test piece was finished by a surface grinding machineNo. 400. Further, the density of the test piece was measured by anArchimedes method, and the relative density was calculated. The surfaceof the test piece was subjected to lapping and etching treatment,whereupon SEM observation was carried out, and image treatment wascarried out to measure the maximum particle diameter of boron carbideand the abundance ratio (area ratio) of boron carbide particles of from10 to 100 μm to boron carbide particles having a particle diameter of atmost 5 μm. The electrical conductivity was measured by means of a fourterminal method.

The results of evaluation are shown in Table 4. The sintered body had arelative density of at least 90%, a maximum particle diameter of at most100 μm, an abundance ratio (area ratio) of the boron carbide particlesbeing within a range of from 0.02 to 0.6, and had an electricalconductivity of at least 5×10² S/m, a four-point flexural strength of atleast 400 MPa and a fracture toughness of at least 3.0 MPa·m^(1/2).TABLE 4 Examples and Comparative Examples Density Relative Abundance B₄Cof density ratio of Maximum Electric CrB₂ starting Sintering sintered ofB₄C particle Flexural Fracture conduc- amount material temp. bodysintered particles diameter strength toughness tivity No. mol % powder °C. g/cm³ body % % μm MPa MPa · m^(1/2) S/m Ex. 1 20 I 2030 2.86 98.10.09 32 528 3.7 2.1 × 10⁴ Ex. 2 20 II 2030 2.84 97.2 0.08 35 460 3.6 1.2× 10⁴ Ex. 3 15 I 2050 2.75 97.6 0.40 75 457 3.1 7.3 × 10³ Ex. 4 22.5 I2020 2.85 95.8 0.26 58 436 3.5 8.6 × 10³ Comp. 20 III 2030 2.57 87.90.01 32 320 2.4 5.5 × 10² Ex. 1 Comp. 7.5 I 2030 2.11 79.5 0.01 16 1752.3 7.5 × 10 Ex. 2

EXAMPLE 6

To a boron carbide powder II having the physical properties asidentified in Table 3, 20 mol% of a chromium diboride powder having anaverage particle diameter (D₅₀) of 3.5 μm was blended, and using amethanol solvent, the blend was mixed by a planetary ball mill made ofSiC at a rotational speed of 275 rpm for 1 hour. The slurry was dried byan evaporator and further dried at 150° C. for 24 hours, whereupon itwas sieved through a sieve of 250 mesh to obtain a boroncarbide-chronium diboride mixed powder.

This powder was molded in a mold under 20 MPa, followed by CIP moldingunder 200 MPa to obtain a molded product. The molded product was putinto a graphite container and placed in a resistance heating type firingfurnace. Heating was carried out at a temperature-raising rate of 40°C./min while vacuuming to a pressure of from 2.0×10⁻¹ to 2.0×10⁻² Pa bymeans of a diffusion pump. When the temperature reached 1000° C.,vacuuming was terminated, and Ar gas was introduced, followed by heatingto 1500° C. From 1500° C. to 2030° C., the temperature was raised at arate of 10° C./min. After the temperature reached 2030° C., sinteringwas carried out for 1 hour under a non-pressurized condition to obtain aboron carbide-chromium diboride sintered body.

The surface of a test piece was finished by a surface grinding machineNo. 400. Further, the density of the test piece was measured by anArchimedes method, and the relative density was calculated. The surfaceof the test piece was subjected to lapping and etching treatment,whereupon SEM observation was carried out and image treatment wascarried out to measure the maximum particle diameter of boron carbideand the abundance ratio (area ratio) of boron carbide particles of from10 to 100 μm to boron carbide particles having a particle diameter of atmost 5 μm. The electric conductivity was measured by a four-terminalmethod.

The results of evaluation are shown in Table 4. The sintered body had arelative density of at least 90%, a maximum particle diameter of at most100 μm, the abundance ratio (area ratio) of boron carbide particlesbeing within a range of from 0.02 to 0.6, and had an electricalconductivity of at least 5×10² S/m, a four-point flexural strength of atleast 400 MPa and a fracture toughness of at least 3.0 MPa·m^(1/2).

Comparative Example 3

Sintering was carried out under a non-pressurized condition in the samemanner as Examples 5 and 6 except that a boron carbide powder III havingthe physical properties as identified in Table 3 was used, to obtain aboron carbide-chromium diboride sintered body, and evaluation wascarried out.

In Table 4, the results of evaluation are shown. In Comparative Example3, a boron carbide powder having an average particle diameter (D₅₀)larger than 2 μm and a specific surface area (BET) smaller than 10 m²/g,was used whereby a dense sintered body was not obtained, and theabundance ratio (area ratio) of boron carbide particles was outside therange of from 0.02 to 0.6, whereby the flexural strength and thefracture toughness had low values.

EXAMPLE 7

To a boron carbide powder I, 15 mol% of the same chromium diboridepowder as in Examples 5 and 6, was blended, and in the same manner as inExamples 5 and 6, a boron carbide-chromium diboride mixed powder wasprepared. Sintering was carried out under a non-pressurized condition inthe same manner as in Examples 5 and 6, except that the sinteringtemperature was changed to 2050° C., to obtain a boron carbide-chromiumdiboride sintered body, and evaluation was carried out.

The results of evaluation are shown in Table 4. The sintered body had arelative density of at least 90%, a maximum particle diameter of at most100 μm, an abundance ratio (area ratio) of boron carbide particles beingwithin a range of from 0.02 to 0.6, and had an electric conductivity ofat least 5×10² S/m, a four-point flexural strength of at least 400 MPaand a fracture toughness of at least 3.0 MPa·m^(1/2).

EXAMPLE 8

To a boron carbide powder I, 22.5 mol% of the same chromium diboridepowder as in Examples 5 and 6, was blended, and in the same manner as inExamples 5 and 6, a boron carbide-chromium diboride mixed powder wasprepared. Sintering was carried out under a non-pressurized condition inthe same manner as in Examples 5 and 6 except that the sinteringtemperature was changed to 2020° C., and evaluation was carried out.

The results of evaluation are shown in Table 4. The sintered body had arelative density of at least 90%, a maximum particle diameter of at most100 μm, an abundance ratio (area ratio) of boron carbide particles beingwithin a range of from 0.02 to 1.6, and had an electric conductivity ofat least 5×10² S/m, a four-point flexural strength of at least 400 MPa,and a fracture toughness of at least 3.0 MPa·m^(1/2).

Comparative Example 4

Sintering was carried out under a non-pressurized condition in the samemanner as in Examples 5 and 6 except that the amount of the chromiumdiboride powder was changed to 7.5 mol%, to obtain a boroncarbide-chronium diboride sintered body, and evaluation was carried out.

The results of evaluation are shown in Table 4. The amount of thechromium diboride powder was small, and no adequate amount of thechromium diboride based liquid phase was formed, whereby a densesintered body was not obtained, and the abundance ratio (area ratio) ofthe boron carbide particles was not within the range of from 0.02 to0.6, the electrical conductivity was not improved, and the flexuralstrength and the fracture toughness had low values.

Industrial Applicability

According to the present invention, the following industrially usefuleffects can be obtained.

-   (1) It is possible to prepare a boron carbide-titanium diboride    sintered body having a high four-point flexural strength of at least    700 MPa.-   (2) It is possible to obtain a boron carbide-titanium diboride    sintered body which has a high density and a maximum particle    diameter of boron carbide of 5 μm, wherein titanium diboride    particles are uniformly dispersed in the boron carbide matrix, the    agglomerated/dispersed state of titanium diboride particles is    uniform and good, and the fracture toughness is improved.-   (3) The boron carbide-titanium diboride sintered body has a    four-point flexural strength as high as at least 700 MPa which has    not been obtained by a conventional method, and it is useful in a    wide range of applications for e.g. sliding components, cutting    tools, bullet-proof plates and new abrasion resistant components,    and is thus industrially useful.-   (4) It is possible to obtain a sintered body wherein a highly    electrically conductive chromium diboride phase forms a network    structure three dimensionally.-   (5) The boron carbide-chromium diboride sintered body of the present    invention can be prepared by heating (sintering) under a    non-pressurized condition at a low sintering temperature.-   (6) The sintered body has a high density and good electric    conductivity and is processable by discharge processing.-   (7)A novel abrasion resistant component can be provided.-   (8) The boron carbide-chromium diboride sintered body has high    strength and toughness and is excellent in mechanical properties,    and thus, it is useful for various applications for e.g. sliding    components, cutting tools and new abrasion resistant components and    is thus industrially useful.

1. A boron carbide based sintered body characterized by having a fourpoint flexural strength of at least 400 MPa as measured in accordancewith JIS R1601 and a fracture toughness of at least 2.8 MPa·m^(1/2) asmeasured in accordance with JIS R1607-SEPB method.
 2. The boron carbidebased sintered body according to claim 1, which is a boroncarbide-titanium diboride sintered body obtained by sintering a mixedpowder of boron carbide (B₄C) powder, titanium dioxide (TiO₂) powder andcarbon (C) powder while reacting them under a pressurized condition andwhich comprises from 95 to 70 mol % of boron carbide and from 5 to 30mol % of titanium diboride, wherein the boron carbide has a maximumparticle diameter of at most 5 μm.
 3. The boron carbide based sinteredbody according to claim 1 or 2, wherein the four point flexural strengthis at least 700 MPa.
 4. The boron carbide based sintered body accordingto claim 1, 2 or 3, wherein the four point flexural strength is at least800 MPa, and the fracture toughness is at least 3.0 MPa·m^(1/2).
 5. Aboron carbide based sintered body which is a boron carbide-chromiumdiboride sintered body containing from 10 to 25 mol % of chromiumdiboride (CrB₂) in boron carbide (B₄C), characterized in that thesintered body has a relative density of at least 90%, boron carbideparticles in the sintered body have a maximum particle diameter of atmost 100 μm, and the abundance ratio (area ratio) of boron carbideparticles of from 10 to 100 μm to boron carbide particles having aparticle diameter of at most 5 μm, is from 0.02 to 0.6.
 6. The boroncarbide based sintered body according to claim 5, which has an electricconductivity of at least 5×10² S/m.
 7. The boron carbide based sinteredbody according to claim 6, which has a four point flexural strength ofat least 400 MPa and a fracture toughness of at least 3.0 MPa·m^(1/2).8. A process for producing a boron carbide based sintered body,characterized by mixing a titanium dioxide powder having an averageparticle diameter of less than 1 μm and a carbon powder having anaverage particle diameter of less than 1 μm to a boron carbide powderhaving a maximum particle diameter of at most 5 μm, an average particlediameter of at most 1 μm and a specific surface area of at least 10m²/g, and sintering the mixture within a temperature range of from 1900to 2100° C. while reacting them under a pressurized condition.
 9. Theprocess for producing a boron carbide based sintered body according toclaim 8, wherein the specific surface area of the boron carbide powderis at least 16 m²/g, and the average particle diameter of each of thetitanium dioxide powder and the carbon powder is less than 0.1 μm.
 10. Aprocess for producing a boron carbide based sintered body, characterizedby molding a raw material powder having from 10 to 25 mol % of achromium diboride powder added and mixed to a boron carbide powderhaving an average particle diameter (D₅₀) of at most 2 μm and a specificsurface area of at least 10 m²/g, followed by heating from 1950 to 2100°C. in a non-oxidizing atmosphere under a non-pressurized condition. 11.A shock absorber made of the boron carbide based sintered body asdefined in any one of claims 1 to
 7. 12. The shock absorber according toclaim 11, wherein the shock absorber is for a high velocity missile. 13.An abrasion resistant component made of the boron carbide based sinteredbody as defined in any one of claims 1 to 7.